BC Rich booster Cap Mod

[bassboy]

I have a stock 1978 BC Rich Eagle Bass with a stock onboard booster circuit by Neal Moser. The stock booster, when engaged, has a bloated tone with the highs muted, compared to the sound with the booster dis-engaged; this is the way that they are apparently. To add more presence to the booster circuit Neal offers a unit with a Cap Mod that adds a DPDT switch wired with a .0047 capacitor to the booster circuit. See diagram. Is this .0047 cap just cutting bass? To my ears I'm assuming the stock booster circuit is boosting volume (5x's available via pot) and EQ'd to have a puffy midrangey sound. Will this .0047 cap cut the mids making the highs and lows more apparent? Also, what other cap values would bring some more high mids/highs back, hopefully more than the .0047 he recommends? Thanks for any help. Keith

[Chuck H]

Edited/deleted this post because Mick interpreted things better in post 3.

Thanks Mick.

[Mick Bailey]

The cap mod inserts the 0.0047uf cap in series between the pickup selector and the input to the booster. Along with the input resistance to the booster it forms a high-pass filter, though the corner frequency can't be determined unless you know the booster's input resistance. Increasing the cap increases mids/bass. Decreasing it reduces mids/bass.

If you wanted to experiment before committing to modifying your instrument you can simply unsolder the pickup selector output lead and either clip-lead or temporarily tack-solder different cap values in until you get one that best suits you. This will give you a good idea of what the mod does when the boost and cap are engaged.

As only one side of the DPDT switch is used, you can use a SPDT version.

If you prefer the modified sound and don't want to fit an additional switch you can just insert the cap into the input lead to the preamp - lift the existing wire off the switch (being careful not to overheat the terminal), solder one leg of the cap to the switch where the wire came from and the other leg to the wire. Insulate the joint with heatshrink or tape.

[Chuck H]

I agree with Mick. If the tone from the booster is unusable without modification you may as well hard wire a capacitor in series with the booster input so it's always there rather than switched in and out. This would make it so you don't need another switch as Mick noted.

[bassboy]

Chuck and Mick, thanks for your time. Besides the .0047 that Moser recommends, what value could I buy to lower the mids/bass even more; could you recommend two or three additional values that I can purchase and try? The tone of the booster circuit is so bloated that the .0047 might not be enough for my taste. The schematic for this booster is not available from Moser, only the diagram. I appreciate any help as I'm merely an owner of a soldering iron who has soldered up connections with no true knowledge of circuits -lol. Best, Keith

[Chuck H]

If you want to cut more bass then a lower value capacitor could be used. Sooo...

These are standard capacitor values in the range you're asking about that you should be able to buy:

.0047uf
.0039uf
.0033uf
.0028uf
.0022uf

This may or may not give you what you want. I can't find details of the Moser booster circuit anywhere. As Mick mentioned it might help to know the input impedance but we can't without schematic information. That said we can guess about it bassed on the .0047uf cap chosen as a bass cut circuit by the designer. So it might be possible to further idealize the input by using a resistor paralleled with a capacitor to give a more scooped sound to the boost. When I think "bloated" I interpret it as the opposite of scooped. So my thinking is that scooping the boost circuit input signal might be even better than just chopping off bass frequencies.?. Anyhoo. Here are some parallel resistor/capacitor values that "I" might try:

150k/.001uf
150k/470pf
150k/330pf
220k/.001uf
220k/470pf
220k/330pf
330k/.001uf
330k/470pf
330k/330pf

In case you didn't notice you'll only need to buy six components to try all the above combinations. Your description of "bloated" makes me think one of these resistor/cap combinations may suit you better.

And just so it's clear any of the above circuit modifications is going to reduce the booster output. Though I think it should still sound "boosted".

Work quick and simple by just tack soldering things in place for the experimenting to save time. Then do a prettier and more secure job once final value/s are chosen.

Good luck and have fun.

[x-pro]

All that remains is to drive the pitchfork through the water.
No matter how he burns out the switch terminals by resoldering.

[bassboy]

Chuck, thanks a bunch. X-pro, Great tip, thanks. I'll solder the testers to a DPDT switch that I have so that I'll maintain the integrity of the existing booster circuit switch.

[bassboy]

Chuck, The booster circuit is definitely adding some puffy mids to the tone, which I'm not needing, so your suggestion of taming some mids is a good one. To me it's more of a low/midrange booster by design. When I initially try the .0047 cap as suggested by the circuit designer, would I add one of 150/220/330k resistors in parallel with the .0047? Or does the addition of a resistor work best in the following examples you offered?:
150k/.001uf
150k/470pf
150k/330pf
220k/.001uf
220k/470pf
220k/330pf
330k/.001uf
330k/470pf
330k/330pf

[Chuck H]

Skip the parallel resistor with the .0047uf cap. All the resistor would do with a cap of that value is reduce the bass cut and trim a little HF. Basically...

A capacitor in series with the signal impedes bass frequencies. The lower the value the less bass frequencies get through. A resistor impedes ALL frequencies.

By using a resistor and capacitor in parallel the resistor can impede all frequencies EXCEPT those allowed through the capacitor which allows these higher frequencies to bypass the resistance. Get it?

So the smaller value capacitors used parallel with the resistor are chosen specifically to allow high mid and treble frequencies to bypass the resistors impedance of the signal. While the bass and low mid frequencies are still being "resisted".

I'm going to add a simple diagram in a little bit that may make this clearer.

[bassboy]

Chuck thanks for this. I'm waiting for the .0047 cap to arrive by mail soon. To set up for this I'm going to have a DPDT switch wired and well away from the bass guitar and circuit. This way I'm not exposing the instrument and circuit to my testing.

[Chuck H]

This is only intended to demonstrate the affect of the discussed circuits and components. No accuracy is claimed. But I think this could be informative for someone inexperienced.

[Chuck H]

Chuck thanks for this. I'm waiting for the .0047 cap to arrive by mail soon. To set up for this I'm going to have a DPDT switch wired and well away from the bass guitar and circuit. This way I'm not exposing the instrument and circuit to my testing.

Well you don't want really long, unshielded leads for this. I suppose you could use instrument cables? No reason I can think of the run long leads except to protect the guitar from any mishaps. I would just dangle the leads for the booster input outside the guitar and tack solder components until I found what I liked. Once I settled on something I would hard wire it that way and put the guitar back together proper. As far as stock circuit integrity,.. You won't be using the stock boosted circuit so it doesn't matter. If you ever sell the guitar it's easy enough to undo the modification. It's just one lead wire.

[bassboy]

Chuck, hanging the wire outside the guitar is what I'll do. Thanks for the diagram, too. I like this forum because there is alot of experience here. No switch and hard wiring with a single wire is the way to go, for sure. I just ordered all the parts and will check back in with my results.

[Mick Bailey]

All that remains is to drive the pitchfork through the water.
No matter how he burns out the switch terminals by resoldering.

That's why I suggested clip leads or soldering to the pickup selector, which stands little chance of damage. The miniature DPDT switch has limited solder/desolder cycles and is easily damaged.